The present invention relates to a method for electrolytically depositing a thin conductive layer of copper on a nonconductive substrate. More specifically, it concerns a method for adherently depositing a thin continuous conductive layer of copper over a layer of electroless metal which has been deposited on the surface of a plastic substrate.
The art of electroplating plastics is relatively well known. For example, chemical processes are commercially available for activating the surface of plastics so that they will conduct electrical current, thereby making electroplating by conventional methods possible. However, even with many improvements in chemical processes and electroplating techniques, there are still conditions which arise in the practice of processing plastic parts which can cause a high percentage of unacceptable parts due to plating defects. One very common defect is a lack of metallic coating (or a void) in the areas surrounding the points where electrical contact is made with the article being plated. This condition is commonly called "burn-off" by those in the plastic electroplating industry. It has been demonstrated that burn-off is caused by inadequate conductivity between the very thin electroless deposit (nickel or copper) that is first applied to the activated plastic surface and the contact points of the racking fixture. This condition becomes more severe as the area of the part to be plated increases, along with its increased number of contact points. It is understandable that, as the area of the part increases, the amperage necessary for adequate coverage of the electrodeposited coating must also be increased. This can only be achieved by an increase in electrical potential (voltage). Therefore, the resistance at each contact becomes increasingly critical as the voltage required for deposition of the first metallic coating increases. The points of higher resistance generate so much heat that the contact bridge is quickly burned through if additional metal is not immediately deposited to reinforce the current-carrying capacity. As contact is lost, the current load is then shifted to the other contact points and burn-off may then become a chain reaction continuing until there is a complete loss of electrical contact, provided sufficient bridging has not occurred by that time. However, even the loss of a single contact point will generally result in rejected work. In such cases, the metallic electroless deposit that surrounds the barren, nonconductive burn-off area becomes bipolar to the highly negative rack contact, and the coating that may have occurred in that area is anodically dissolved with the final result being a much larger area that is void of any metallic coating. It is also common knowledge that areas of high stress, created during the molding of the plastic parts, are difficult to etch, resulting in thin, electroless deposits which are incapable of conducting the surge of current when the potential is applied to begin electroplating and therefore a contact point in any such area will generally result in electrical failure.
Prior art techniques, in general, suffer from the following deficiencies: (a) their susceptability to burn-off and the lifting off of the plate from the plastic substrate; (b) the high cost of operation because of rejected parts or lost production; (c) the higher cost of operation when using a nickel strike instead of a copper strike; (d) the poor throwing power and low current density coverage of the electroless metal deposits by nickel and copper strikes, particularly around and under the contacts; (e) the coarse grain structure and objectionable appearance of deposits from the strike baths which do not contain additives; and (f) the high voltage requirements needed for strikes of the present art in order to achieve metal deposition in areas of extreme low current densities.
Accordingly, it is the main object of the invention to provide an improved process for the electroplating of plastic parts.
A further object of the invention is to provide an improved process for the electroplating of plastic parts which eliminates the common problem of "burn-off" which results in a large number of rejected parts in production installations.
A still further object of the invention is to provide a process for electroplating plastics which includes a novel copper strike with both superior throwing power and coverage so as to form a satisfactory deposit in the recessed areas and around contact points.
Another object of the invention is to provide additives for the copper strike which influence the characteristics of the copper deposit. This includes the production of a bright, fine-grained deposit with excellent ductility and low stress.
A still further object of the invention is to produce a copper strike that will give the desired low current density coverage and protection of thin electroless deposits at voltages below those which produce "burn-off".
These and other objects of the invention will become apparent to those skilled in the art from a reading of the following specification and claims.